Some poskim say that when a person who is accustomed to one nusach goes to pray in a minyan held in a different nusach, he must pray according to the nusach of the minyan he is attending, because the individuals must follow the majority. If he practices according to his own minhag in front of the people in the congregation, it constitutes a transgression of the prohibition “lo titgodedu” (fragmenting the nation into divergent groups). This prohibition disallows having one court of law (beit din) with some judges who rule according to the method of Beit Shamai and others who rule according to Beit Hillel, so that the Torah will not be divided into two seemingly different Torahs (Yevamot 14a, according to the Rif and the Rosh). Hence, the people of one synagogue should not pray in two different nusachim. Furthermore, the Chachamim teach (Pesachim 50b) that a person must not stray from the custom of a place so as not to generate dispute (Pe’at HaShulchan 3:14).
According to most poskim, one is permitted to pray in his family’s nusach those parts of the prayer service that are recited silently. Since the differences are not noticeable, there is no fear of dispute, nor is there any transgression of the prohibition, “lo titgodedu.” However, prayers recited aloud should be prayed in the nusach of the minyan so as not to create controversy and disparity among the members of the congregation.
One who must regularly pray in a minyan conducted in a different nusach, e.g., because he moves to a place in which the only minyan prays in a different nusach or because he prays in the minyan that will strengthen his religiosity, is permitted to decide whether to pray in its nusach, or adhere to his own family’s custom, reciting the parts said aloud like the congregation.
The chazan leading the prayer service in a synagogue employing a nusach that is different from his own must pray according to the minhag of the place because he is praying as the people’s emissary. However, for the silent prayers, he may pray according to his own minhag.
.The prayers recited silently may be prayed in one’s own nusach. However, the Kedushah, which is recited out loud, should be prayed in the chazan’s nusach, as written in Shut Sho’el U’Meishiv Edition 3, 1:247, Meishiv Davar 1:17, Shivat Tzion 5, Igrot Moshe, Orach Chaim, part 2, 23, and Minchat Yitzchak 7:5. Regarding Pesukei d’Zimrah, Birkot Keriat Shema, and Nefillat Apayim, the poskim are uncertain. According to the Igrot Moshe, Orach Chaim, part 2, 23, since one is permitted to say Pesukei d’Zimrah out loud, it is better that he says them in the chazan’s nusach. Similarly, concerning the matter of Nefillat Apayim on Mondays and Thursdays, there is a difference in custom. According to Nusach Sephard, we put our heads down before Tachanun, and according to Nusach Ashkenaz, after Tachanun. The Igrot Moshe, Orach Chaim, part 3, 89, writes that one may not stray from the custom of the place because it would be noticeable. However, the widespread custom is that every person prays according to his minhag. Since it is known that there are many different minhagim in prayer, the prohibition of “lo titgodedu” is not applicable in this case. Yet, those praying in a nusach different from that of the chazan should not call attention to themselves by praying out loud, as mentioned in Tefillah Kehilchatah 4, notes 23 and 26. However, the Yabia Omer, part 6, 10, maintains that even prayers which are recited out loud should continue to be prayed by each person in his own nusach and there is no fear of “lo titgodedu” nor controversy because everyone knows that different minhagim exist. Nonetheless, even according to his opinion, it seems that one should not actually recite those prayers aloud, because if he does, he incites controversy.
In practice, the person praying is permitted to choose a nusach – his or the chazan’s – however, for the more noticeable prayers, it is proper to pray according to the chazan’s nusach. Further, see Igrot Moshe Orach Chaim, Part 4, 34, outlining the aspects of the prohibition to pray according to different minhagim in one synagogue.